Multiple Sclerosis (MS), an autoimmune disease of the central nervous system (CNS), is the most common non-traumatic cause of neurological disability among young adults in the United States. Exacerbations of the disease occur when inflammatory foci form in the optic nerves, brain or spinal cord, resulting in demyelination and damage to neighboring axons. A critical step in the formation of MS lesions is adhesion molecule mediated transmigration of leukocytes from the bloodstream into the CNS. The current proposal tests our hypothesis that targeted blockade of a particular adhesion molecule interaction (between a4b7, expressed on CNS-infiltrating T cells, and MAdCAM-1, expressed on inflamed CNS blood vessels) will prevent exacerbations of autoimmune demyelination in a mouse model of MS (experimental autoimmune encephalomyelitis or EAE). To do so, we have engineered a chimeric fusion protein (MAdCAM-1-Fc), composed of the extracellular domain of MAdCAM-1-Fc linked to the Fc region of mouse immunoglobulin heavy chain. This reagent prevents MAdCAM-1 from binding a4b7 expressing T cells. We will determine whether systemic administration of MAdCAM-1-Fc or control fusion proteins suppresses relapses or progression of EAE. Furthermore, in a test of its safety, we will administer MAdCAM-1-Fc to mice infected with an alphavirus and determine whether the treatment impedes viral clearance and/ or triggers reactivation of latent virus in the CNS. The specific aims of our proposal are as follows: 1. To study the temporal and spatial expression of a4b7 on CNS infiltrating CD4+ T cells and its cognate ligand, MAdCAM-1, on CNS vascular endothelium during the course of relapsing and chronic experimental autoimmune encephalomyelitis (EAE) in mice. Additional experiments are designed to examine the inflammatory factors that induce expression of a4b7 on myelin-specific effector cells and MAdCAM-1 on CNS endothelial cells. 2. To investigate the therapeutic consequences of a4b7 blockade in both relapsing and chronic EAE models. We will compare the therapeutic efficacy of a novel MAdCAM-1-Fc recombinant fusion protein (that specifically blocks a4b7 integrin) versus other adhesion molecule blocking agents, when given at different time points in the disease course. 3. To characterize the effects of systemic a4b7 blockade on acute viral infection of the CNS and reactivated latent viral infection in mice. We will use a mouse model of alphavirus infection to compare the effects of MAdCAM-1-Fc and other fusion proteins on anti-viral immunity and immunosurveillance.